The goal of our research effort is to understand the regulation of hormone responsiveness during cellular differentiation. Acquisition of hormone sensitivity often accompanies differentiation, therefore, by understanding its regulation, we will better understand the process of differentiation. In order to allow better control of experimental conditions, a model system using cultured cells was established. We found that in a dog kidney cell line, MDCK cells, glucagon responsiveness was selectively lost after transformation by Harvey murine sarcoma virus. This loss of hormone sensitivity can be restored to the transformed cells by culturing the cells in the presence of prostaglandin E2. The induction by PGE2 seems to be mediated by cyclic AMP. We also found that this induction process is inhibited by a serum factor, epidermal growth factor and a phorbol ester (TPA). Using these differentiation inhibitors, we are attempting to define the nature of this cyclic AMP-dependent process. It is apparent that the inhibitory effect of EGF resides downstream beyond the activation of cyclic AMP-dependent protein kinase. In the presence of PGE2 during induction, EGF receptors undergo a biphasic regulation. EGF binding increases initially and reaches a peak in 18 hr, then followed by a decrease to less than 20% of the maximal binding in 48 hr. The induction and the subsequent desensitization of EGF receptors suggest a cyclic AMP-dependent modulation of EGF effect. Using a semi-purified preparation of EGF receptors, we are currently studying their phosphorylation by both EGF- and cyclic AMP-dependent processes. In addition, we also found that the ras viral protein p21, production is decreased when cells are induced to differentiate. SInce a role in signal transduction has been suggested for p21, we are also examining the potential interaction between the viral protein and receptors for growth factors.